BACKGROUND: Dementia with Lewy bodies (DLB) is now considered to be the second most common pathological cause of dementia after Alzheimer's dementia (AD). Little has been known about differences in the neuropsychological assessment between DLB and AD. The aim of this study is to assess the pattern of cognitive impairment in DLB and to differentiate DLB from AD using neuropsychological tests. METHODS: Three subject groups (11 DLB patients, 16 AD and 14 normal) participated in this study. They were all matched for age and education period. They were diagnosed as probable DLB and AD according to the clinical criteria of the consortium on DLB and NINCDS-ADRDA. All patients were evaluated by a neuropsychological battery of tests. RESULTS: Compared with the normal control group, DLB and AD patients demonstrated cognitive decline with significant attentional deficits, frontal executive and visuospatial dysfunctions, memory dysfunctions and impairment on language and related functions (p<0.05). Neuropsychological tests revealed no statistically significant differences between DLB and AD. However, DLB patients performed worse than AD patients on several cognitive domains of frontal executive function (semantic and phonemic fluency), and visuospatial function (copy of Rey figure). On the contrary, DLB patients performed better than AD patients on tests of verbal delayed recall. CONCLUSIONS: Although DLB patients showed significant cognitive declines comparable to AD, neuropsychological tests revealed a somewhat different pattern of cognitive impairment in DLB patients compared with AD. It is suggested that neuropsychological testing may be helpful in differentiating DLB from AD.